"Your lifestyle will mess a kid up" is such a bullshit complaint. Child abuse and neglect will mess a kid up. Growing up in poverty will mess a kid up. Not loving a kid will mess a kid up. Living in a household that's gay or poly or Sri Lankan or Baha'i will only mess a kid up if you consider those things and tolerance thereof to be "messed up". Whether you realize it or not, your complaint is not about children and their welfare, it's about the propagation of an adjective you consider suboptimal.

Parents have a natural tendency to hide their sexual sides from their children - which is certainly a good thing - but I think it contributes to the perception that the older generation is more socially conservative and more sexually uptight. I assure you, the generation that invented the term "polyamory" was not the first to practice it, any more than the "free love" generation invented promiscuity. Society changes over time, human nature.... well, okay, human nature changes too, but much less so.

ChA: What about kids, you're not planning on ever having kids either, I'm guessing? Or are you?
CA: Trent and I have discussed it a few times, but we're not really thinking about that at this point in our lives. I mean, I'm twenty-six, I've got time.
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ChA: Yeah, but... I mean how the hell do you raise a kid with a household like that?
CA: Same way you do in any other household: with love and honesty and responsibility and attention. The kids don't need to know the specifics about Mommy and Daddy's love lives until they're older, and by then, they likely won't want to know.
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ChA: You don't think finding out about Mommy and Daddy's other lovers is gonna mess a kid up?
CA: Well, how do you feel about Miguel?
ChA: Uncle Meegs? That's different. He's just Mom and Dad's friend, nothing weird or polyamorous about that. He just moved in with us for a while to help out with child care after Carl was born.
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ChA: ...our little brother Carl, who has black hair and brown eyes and evidently my entire life is an elaborate web of deception.
CA: Omission! An elaborate web of omission!